1. Field of the Invention
This invention relates to a technique for obtaining metals by reductive deposition process, more particularly, to a process for obtaining noble metals by the chemical reduction by allowing an aqueous solution of a noble metal to contain a --SH or --S-- containing compound as a reducing agent.
2. Description of the Prior Art
Chemical reduction and deposition of metals from their aqueous solutions with the aid of a reducing agent has been utilized in the method of obtaining metal coatings known as autocatalytic electroless plating and also in the process for producing (fine) metal particles for catalyst and other applications.
Reducing agents thus far reported for use in depositing metals from their solutions have included: e.g., for the deposition of gold, borohydride, dimethylamine borane, phosphinic acid, hydrazine, hydroxylamine, hydrazine boron, thiourea, ascorbic acid, titanium trichloride, formaldehyde, tartaric acid, glyoxylic acid, and formic acid; for platinum, hydrazine; for silver, glucose, formaldehyde, dextrin, glyoxal, ascorbic acid, sorbitol, hydroxylamine, hydrazine, borohydride, and dimethylamine borane; and for palladium, hydrazine, phosphinic acid, and trimethylamine borane (all inclusive of their salts).
Deposition can take place with reducing agents other than those mentioned above for the respective metals. However, gold, platinum, silver, palladium, etc. are noble metals, and the deposition potentials being noble, they are easily reduced and hence their deposition rates are rather difficult to control. There has generally been a contradictory fact that from the standpoint of productivity a high deposition rate is desirable but the conditions that permit fast deposition tend to make the solution and hence the complex unstable.
With these in view, the present invention is aimed at developing novel reducing agents f or the deposition of noble metals by chemical reduction and also developing aqueous solutions from which noble metals can be obtained by chemical reduction, the solutions remaining stable under the conditions that make high rates of deposition possible.